i mean, technically you're right.
the entire antitrust lawsuit is premised on the notion that the UFC has monopoly power, and the ability to outbid other promotions on all top fighters, and thus maintain a monopoly on ranked fighters in the sport.
by actually signing all of the top fighters, and outbidding all other promotions, this is illegal monopolistic behaviour according to the lawsuit, and should result in the UFC having to pay out a billion dollars.
By letting other promotions outbid the UFC on certain top fighters, the UFC is actually helping their cause to say they clearly aren't a monopoly.
So yes, they can't afford all the top fighters -- not in the sense that they don't have the money, but because if they do actually sign all the top fighters, it's proof of their monopoly. (At least according to the lawsuit).
If you don't believe me, you should read the actual lawsuit:
https://angeion-public.s3.amazonaws.com/www.FighterClassAction.com/docs/ECF+No.+1+-+Complaint.pdf
138. While skilled Professional MMA Fighters may emerge outside of the UFC or break off from the UFC, those Fighters cannot demonstrate their skill, garner attention, or otherwise maintain sustainable careers outside of the UFC. The measure of success of a Professional MMA Fighter is dependent upon the level of competition he faces and his success or failure when doing so. The success of an Elite Professional Mixed Martial Artist requires that he or she register wins over fighters seen by the viewing audience and media as Elite Professional MMA Fighters in widely viewed MMA events to build public notoriety, reputation, fan base, sponsor interest and earnings potential. Professional MMA Fighters who compete at the highest level of the sport cannot “opt out” of UFC because the UFC’s anticompetitive conduct has made it impossible to maintain a successful MMA fighting career outside of the UFC.
139. Likewise, because UFC Fighters are bound by non compete agreements, and because the UFC will not co promote, would be rival MMA promotion companies cannot stage bouts between their own non UFC fighters and UFC Fighters. Because the UFC Fighters are considered MMA’s Elite Professional MMA Fighters, would be rival MMA promotion companies cannot compete effectively. Without big ticket MMA Cards with Elite Professional MMA Fighters, would be rival promotions are unable to secure sufficient public interest or sponsors and venues large enough or prestigious enough to generate revenues and bout purses that can sustain the demands of training costs, travel, health coverage, gym membership, sparring partners, and other expenses necessary for sustaining a career as an Elite Professional MMA Fighter. As a result, would be rival promoters do not and cannot promote MMA events that offer Elite Professional Mixed MMA Fighters substantial earnings potential on PPV broadcasts, major network or subscription based broadcast outlets.